[RE-wrenches] Other's thoughts on Autonomy? was concordbatteries, EQUALIZE Them!

Tue Dec 1 16:38:01 PST 2009

Hello Travis,

I think that 2 days is not enough battery autonomy in the Ozarks and many other locations unless you have a fossil fuel generator to carry you through long cloudy periods. Battery autonomy is site and load specific. I've done systems with as little as 1 day and as much as 3 weeks at 80% depth of discharge. Lately, I've been generically specing 1.5 days of autonomy at 50% d.o.d. to get the dialogue started with the customer.

I use to spec up to 4 parallel strings of T-105s or L-16s in 2, 4, and 8 batteries in series, but now I keep the number of strings down to 3 or less and prefer 1 or 2 strings of big 2-volt cells to reduce the number of cells and connections.

Southern California urban and suburban grid-tie PV systems are almost all non-battery although we still get asked about emergency power - until they hear how much it adds to the cost of a grid-tie PV system.

Joel Davidson
  ----- Original Message ----- 
  From: Travis Creswell 
  To: 'RE-wrenches' 
  Sent: Tuesday, December 01, 2009 2:15 PM
  Subject: Re: [RE-wrenches] Other's thoughts on Autonomy? was concordbatteries, EQUALIZE Them!

  IMHO, one of the worst design boo-boo's is going past more then 2 days of autonomy.  Personally, I no longer size much over one day because it's my anecdotal observation that most batteries die of old age and being ignored long before cycles get them.  Speaking mostly about quality deep cycle flooded.

  Lots of good things result;

  -50%-75% smaller battery bank means a $20,000 battery bank just turned into $5,000 bank which frees up a ton of money for more modules and now-a-days you can buy a lot more PV with that money.  More array mean far less reliance on autonomy.  I'll take the trade all year long.  In the summer we have 3 to 4 weeks of sun and one day of clouds and in the winter we get 3 to 4 week stretches with 1 sunny day.  Autonomy doesn't really matter in either case from what I've seen.  The larger the bank means more self discharge losses, which on large battery banks gets significant as they age.  5-15 years later you'll still have all that array but no matter what you're looking at new battery bank.

  -If you study the quality deep cycle manufacturers literature you'll see that you'll see that anything over 1 day of autonomy is too much to allow the array to actually charge the battery bank anywhere near the recommended amps and just like rust, sulfation never sleeps.

  -Less cells to water

  -Less space required

  -Given that a surprisingly high percentage of off gridders totally screw up on their first bank, no matter how much we all try we might as keep the stupid tax of replacing a 2.5 yr old battery bank to a minimum.

  -All of this discussion about cross paralleling, buss bars, TLC with a gazillion connections and multiple strings goes away.

  -And the best part is we don't have to carry all of the lead into the basement and even better back out of the basement!

  Just my .02.  Feel free to strongly disagree but let's be polite about it.

  Travis Creswell

  Ozark Energy Services

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  From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of R Ray Walters
  Sent: Tuesday, December 01, 2009 12:44 PM
  To: RE-wrenches
  Subject: Re: [RE-wrenches] concord batteries, EQUALIZE Them!

  I used to think that one string was optimal; until I had a single cell failure take out an entire system for weeks. (try operating a 24 v system at 22v! )

  I now think that 2 parallel strings is optimum,  3 is OK, and 4 is max.

  At 4 parallel strings, we start spending more time looking to make sure all connectors are the same exact length etc. to insure equal operation.

  But of course how do you account for varying internal resistance of the batteries......??

  I've done 4 parallel strings at 144 DC of sealed batteries on an electric vehicle, but we were very careful with our resistances, I even switched to smaller wire, on closer strings, and calculated out the exact resistance, so all strings were theoretically equal. This set actually just died, but achieved its manufacturer's predicted cycle life. (B&B battery, 350 cycles to 80% DOD)

  So if you're careful, 4 strings can work well.

  Worst I've seen was 20 golf carts paralleled in a 12 v system, (10 strings) and they didn't pull the main connections from across the set, just connected to one end.

  The results were very predictable, with the furthest batteries being chronically under charged, and the closest ones being over cycled to a premature death.

  Ray Walters

  On Dec 1, 2009, at 11:28 AM, Windsun at wind-sun.com wrote:

  You gotta wonder about why the customer bought such a battery layout, or why the installer sold that kind of configuration (which ever it was) with so many small batteries. We would never recommend going over 2 parallel banks, but sometimes the "customer knows best...".

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